1. Field of the Invention
The present invention relates to an H-bridge circuit in which the impact on peripheral circuitry of regenerative current occurring at transistor switching is reduced.
2. Description of the Related Art
H-bridge circuits including four transistors are commonly used for motor control. Referring to FIG. 1 the basic H-bridge circuit has a power supply terminal 1, a ground terminal 2, an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET) Q1 and an n-channel MOSFET Q2 connected in series between the power supply terminal 1 and the ground terminal 2, and an n-channel MOSFET Q3 and an n-channel MOSFET Q4 connected in series between the power supply terminal 1 and the ground terminal 2. MOSFETs Q1 and Q3 are referred to as the upper arms of the H-bridge, and MOSFETs Q2 and Q4 as the lower arms. MOSFETs Q1, Q2, Q3, Q4 inherently include respective parasitic diodes D1, D2, D3, D4.
An inductive load, more specifically a coil load 5 is connected across the node 3 between MOSFETs Q1 and Q2 and the node 4 between MOSFETs Q3 and Q4. For simplicity, the coil load 5 is shown as a single coil. The MOSFETs Q1, Q2, Q3, and Q4 are connected through the coil load 5, forming an H network.
When MOSFETs Q1 and Q4 are turned on by their gate signals, current flows through the coil load 5 from node 3 to node 4, as indicated by the dotted arrow in FIG. 1. This current is referred to as forward current since it turns the motor in the forward direction. The motor speed can be adjusted by a pulse width modulation scheme in which MOSFET Q1 or Q4 is repeatedly switched on and off. When MOSFETs Q3 and Q2 are turned on by their gate signals, reverse current flows through the coil load 5. The reverse current flow can be used to brake or reverse the motor.
When MOSFET Q1 is switched from the on state to the off state, due to a well-known characteristic of coils, current continues to flow through the coil load 5 in the same direction. This continuing current is referred to as regenerative current. Electrons carried out of the coil load 5 by the regenerative current have no place to go and are stored at node 3, as illustrated in FIG. 2. As a result, the drain of n-channel MOSFET Q2 is negatively biased.
Consider the case in which a motor controller including the above H-bridge circuit is formed as a pn junction isolated semiconductor integrated circuit. In this semiconductor integrated circuit, the H-bridge circuit operates as an output stage for peripheral circuits disposed in separate pn junction isolated islands on the same chip. When regenerative current occurs, although the source of MOSFET Q2 is at the lowest potential supplied to the integrated circuit, which is normally the potential of the p-type semiconductor substrate in which MOSFET Q2 is formed, the drain of lower-arm MOSFET Q2 becomes biased at a still lower potential.
The n-type drain region of MOSFET Q2 is surrounded by regions of the opposite conductive type (p-type). When the drain of MOSFET Q2 is negatively biased, the pn junctions between the n-type drain region and adjacent p-type regions becomes forward biased. The adjacent p-type regions may include the p-type body region of MOSFET Q2, the p-type semiconductor substrate, and p-type isolation diffusion regions provided for pn junction isolation. The forward bias between these p-type regions and the n-type drain region permits current to flow toward the drain of MOSFET Q2.
When this current flows from the p-type semiconductor substrate (through parasitic diode D5), parasitic npn transistors (e.g., Tr1) in which the substrate functions as a p-type base layer may turn on, supplying parasitic current to the drain of MOSFET Q2 from peripheral circuits disposed as in separate islands in the same chip. The integrated circuit was not designed for this flow of parasitic current. The unanticipated parasitic current flow may alter supposedly fixed potentials in the islands in which the peripheral circuits are formed, causing unexpected current to flow through the peripheral circuits, leading to circuit malfunctions. A large parasitic current may turn on a parasitic thyristor, causing latchup, which may destroy the integrated circuit.
Similar problems can occur when MOSFET Q3 is switched from the on state to the off state, creating regenerative current that negatively biases the drain of MOSFET Q4.
The general method of solving the problems caused by regenerative current has been to put as much distance as possible between the lower-arm MOSFETs Q2 and Q4 of the H-bridge circuit and the peripheral circuits formed in the same chip. By separating the lower arms and the peripheral circuits, the direct current gain (hFE) of parasitic npn transistors involving the p-type semiconductor substrate can be lowered to reduce the amount of current drawn from the peripheral circuits.
Circuit configurations that prevent the occurrence of parasitic current when the energy stored in the coil is released have also been proposed. In Japanese Patent Application Publication No. H8-223993, for example, Tominaga proposes a motor controller having regenerative diodes at both ends of the coil in the H-bridge, to feed regenerative current stored in the coil back to the power supply, and having capacitors for storing the regenerative current.
In Japanese Patent Application Publication No. H5-236797 (Now Japanese Patent No. 2974188), Kubotsuka proposes an H-bridge circuit in which a pair of recirculating diodes connected between the two ends of the coil and ground allow energy stored in the coil to escape. To keep the recirculating current from flowing into the current detecting resistor through the parasitic diodes in the lower arms of the bridge, power Darlington transistors, which do not form parasitic diodes, are used in the lower arms.
The conventional practice of distancing the lower arms from the peripheral circuits can mitigate the effect of regenerative current on the peripheral circuits but cannot fundamentally eliminate the effect. One alternative solution would be to isolate the element islands with dielectric regions or layers and block all the current that would otherwise flow through junction isolation regions and the substrate. However, fabrication of such a complete blocking structure would be complex, greatly increasing the cost of the integrated circuit.
Taking stored energy directly from the coil load as proposed by Tominaga and Kubotsuka also complicates the structure of the H-bridge circuit because, for one thing, the circuit must be designed so that the regenerative or recirculating diodes placed at the ends of the coil do not conduct current in normal operation.